This invention relates to inflatable drag reducers for land vehicles and, more particularly, to inflatable bags for specifically reducing base drag on land vehicles.
At cruising speeds a great deal of energy is expended to overcome the aerodynamic drag of land-borne transport vehicles. Until recently the solution to this problem for highway vehicles was to install more powerful engines which used more fuel. The recent steep escalation of energy costs has made this approach less desirable. Conversely, reduced aerodynamic drag offers worthwhile savings in propulsion energy. In addition to energy savings, reduced aerodynamic drag will also reduce aerodynamic disturbance to other vehicles, improving operating safety; minimize buffeting, sidesway, and tire wear, reducing costs; and reduce formation of splash and spray during operation on wet pavement, again with safety benefits.
The ideal shape for low aerodynamic drag is a streamlined or teardrop shape like an airplane wing. Most transport vehicles have evolved from rather slow moving conveyances. Therefore, aerodynamic drag was not an important design consideration compared to structural strength and interior cargo volume. Commercial vehicles are also subject to legal and right-of-way limits on overall dimensions and weight. For these reasons, a fully "streamlined" vehicle has not been considered practical or feasible, and basic box shapes have been accepted.
The drag of a basic transport vehicle can be reduced by making the box shape appear aerodynamically more streamlined. Typically, the total aerodynamic drag on a van-type transport vehicle is composed of about 70% forebody drag, about 20% base drag and about 10% skin friction drag.
Forebody drag, caused by flow separation about the sharp corners and projections near the front of a boxy vehicle, is the largest component of drag. Since a fully-streamlined nose is impractical for size and weight reasons, effort has been concentrated toward add-on or auxiliary devices designed to provide smoother flow over the front of the vehicle. Some useful devices have appeared that are low in cost, light in weight and that do not increase the overall dimensions of a vehicle. Such devices can provide reductions in aerodynamic forebody drag of up to 20%. However, it appears unlikely that any significant additional reduction in forebody drag could be achieved by using add-on devices. Larger reductions in forebody drag can be attained with modifications to vehicle design that result in a more streamlined configuration, but they must be included at the factory.
Given such reductions in forebody drag, base drag which results from the low pressure separated wake behind a vehicle becomes an important contributor to the total aerodynamic drag. However, existing drag reduction schemes have ignored base drag and been designed to reduce only the forebody component of aerodynamic drag. Thus, they cannot achieve the maximum reduction attainable under no-wind conditions by methods that affect base drag as well.
The object of this invention, therefore, is to provide an efficient system for reducing base drag on conventionally constructed transport vehicles.